How is IfcCostSchedule related back to IfcProject?

It is ambiguous to me how an IfcCostSchedule is related back to an IfcProject.

One possible suggestion is that an IfcCostSchedule may only be related back through an IfcWorkPlan, but the docs are ambiguous on this scenario.

Can anyone clarify this?

This is an educated guess: Either trough assignment to an object (IfcRelAssignsToControl) or - if the IfcCostSchedule is not bound to an object, but “free-floating” -, then through declaration to the project (IfcRelDeclares).

Any confirmation or correction is appreciated.

Agreed @tauscher that both those suggestions sound plausible. If it is the former, it would be good to clarify what object it should relate to, as well.

Looking forward to further clarification.